For reliable operation and management of an ultra-high-capacity WDM optical communication network, close monitoring on the transmission performance and characteristics of the system is essentially required. OSNR is defined as the ratio of the power of optical signal to that of noise signal being transmitted in a certain optical signal band. Exact measuring of ratio makes it possible to diagnose the performance of the optical transmission system.
In a conventional OSNR measuring method, OSNR is measured by linearly interpolating the intensity of an amplified spontaneous emission (ASE) noise in an optical signal band based on the intensity of the ASE noise in other bands outside the optical signal band (H. Suzuki and N. Takachino, Electronics Letter Vol. 35, pp. 836–837, 1999). In the linear interpolation method, the ASE noise of an optical signal band is estimated to be equivalent to the constant ASE noise of the other bands. Referring to FIG. 1, the ASE noise of each signal band is estimated to be at the level shown with dotted lines. Accordingly, the OSNR of each signal band is determined based on thus estimated ASE noise.
As shown in FIG. 2, however, in a WDM optical transmission system where respective optical signals may pass through different optical paths and through different numbers of erbium-doped fiber amplifiers (EDFAs), the ASE noise included in each optical signal band may vary from band to band. The actual ASE noise included in the optical signal bands of such transmission system should be different from the ASE noise, which was estimated by the linear interpolation method as shown in FIG. 1. Therefore, it is impossible to correctly measure the OSNR by using linear interpolation method.
In order to solve the above problem, Korean Patent No. 341825 entitled “OSNR Monitoring Method and Apparatus Using Polarization Nulling Method” discloses a method of measuring OSNR by polarization nulling method, which utilizes the polarization characteristics of the optical signal and of the ASE noise. A conventional OSNR measurement using the polarization nulling method will be described below with reference to FIG. 3. A wavelength division-multiplexed optical signal including an ASE noise is demultiplexed by a waveguide diffraction grating 11. An optical signal having a polarization characteristic sequentially passes through a quarter-wave plate 12 and a linear polarizer 13. For example, if the quarter-wave plate 12 rotates at a rate of 15 Hz and the linear polarizer 13 rotates at a rate of 0.1 Hz, the minimum power and the maximum power of the optical signal may be measured based on the polarization characteristics of the optical signal including the ASE noise. When the polarization state of the linear polarizer 13 and the polarization state of the optical signal being output from the quarter-wave plate 12 conform to each other, the power of the measured optical signal is maximized. On the other hand, when the polarization state of the linear polarizer 13 and the polarization state of the optical signal being output from the quarter-wave plate 12 are orthogonal to each other, the optical signal component is blocked and only the ASE noise component is outputted. Thus, the power of the measured optical signal is minimized.
The optical signal that has passed through the linear polarizer 13 is converted into an electrical voltage by means of a photo detector 14. Then, the voltage is amplified by a log amplifier 15 and is transmitted and displayed on an oscilloscope 16. A computer 17 calculates the minimum power and the maximum power from the voltage displayed on the oscilloscope 16. From the maximum and the minimum power, the computer 17 calculates the signal power and its ASE noise and then determines a signal to noise ratio of the optical signal.
A conventional OSNR monitoring method employing only polarization nulling method assumes that the polarization nulling function of the linear polarizer 13 is ideal, that is, the nulling ratio is infinite. The method, however, does not consider a case where the polarization nulling ratio of the linear polarizer 13 is finite. Further, the conventional OSNR monitoring method does not consider a phenomenon that the polarization of the optical signal is disturbed due to polarization mode dispersion and non-linear birefringence of the optical fiber. Accordingly, the conventional OSNR monitoring method has a limited use.
Polarization mode dispersion means the time difference between two signal components traveling along two polarization axes orthogonal to each other generated according to the polarization characteristics of the optical fiber or the optical device. Since such polarization mode dispersion is sensitive to environmental variations such as temperature and external pressure, it may vary with time.
Further, the non-linear birefringence is a birefringence occurring due to a change in the refractive index of the optical fiber according to the intensity of the optical signal. When a plurality of optical signals is simultaneously transmitted through one optical fiber, the non-linear birefringence rapidly changes the polarization states of the adjacent channels. Since the change of polarization varies depending on the polarization states between the channels, the resultant effects of the non-linear birefringence also varies with time.
Furthermore, in a conventional OSNR monitoring method employing only the polarization nulling method, an unpredictable error tends to occur due to the polarization mode dispersion and the non-linear birefringence. In other words, an optical signal with a polarization component orthogonal to the polarization state of the linear polarizer 13 is not completely removed when passing through the linear polarizer 13. It is because that the polarization nulling ratio of the linear polarizer 13 is finite (i.e. not ideal) and the optical fiber causes polarization mode dispersion and non-linear birefringence of the optical signals. However, since the conventional OSNR monitoring method does not consider these problems, the accuracy of OSNR measurement is lowered.
In addition, the conventional OSNR monitoring method employing only the polarization nulling method should have the waveguide diffraction grating 11 for demultiplexing of WDM optical signals and an OSNR monitoring apparatus for each of demultiplexed optical signals. Therefore, the cost of the equipment becomes high.